On this day (Feb 25) in 1570, St Pius V issued the papal bull Regnans in Excelsis declaring Elizabeth I to be a heretic and claimed to release all her subjects from any allegiance to her. It also declared any subject of Elizabeth who was loyal to her excommunicated from the Church of Rome. The name of the bull is taken from the first three words of its text, written in Latin and meaning "ruling from on high" (a reference to God).
While we're at it, can someone tell me the difference between a Bull and an Encyclical?
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